DNS-OARC 24th Workshop, Buenos Aires, Presentations
Upward Referrals Considered Harmful
Note: you can skip ahead to BIND fixes.
Recently the hosting company ISPrime became the victim of a DNS-based DDoS attack using spoofed source addresses. Some are calling it an amplification attack because the query ". IN NS" is quite small (47 octets) while an upward referral response is a bit larger (256 octets). For some additional information on this attack, see ISC SANS story #5713. OARC members can track this as incident #10780.
One interesting aspect of this attack is that the queries are apparently sent to authoritative nameservers only. In the past we have seen DNS-based attacks bounce queries through open resolvers. Its not clear why this attack is using authoritative nameservers. Perhaps because it is easy to get a list of nameservers if you already have a list of domain names and you can be reasonably sure that an authoritative nameserver will give some kind of answer.
The attack brings an old debate back into the light: What is an authoriative nameserver's appropriate response to a query that cannot be answered?
The configuration and/or implementation of some authoritative nameservers causes them to return an upward referral to the root zone. We recommend against this behavior for a number of reasons:
We feel that a REFUSED response code is better than an upward referral.
To find out if your authoritative nameservers return an upward referral, send them a query with the dig program:
$ dig @ns.example.net . NS
If you see a long list of ROOT-SERVERS.NET names and addresses, you have an upward referral. Click here to see an example.
Eliminating Upward Referrals
Submitted by admin on Thu, 2009-01-22 00:06 categories [ ]
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